UNDERSTANDING THE FLUVIAL-TIDAL TRANSITION ZONE IN MODERN AND ANCIENT SYSTEMS USING MAJOR AND TRACE ELEMENTS, C/S ANALYSIS, AND PALYNOLOGY

The fluvial-tidal transition zone (FTTZ) represents a complex depositional environment characterized by the dynamic intermingling of terrestrial and marine sedimentologic, biologic, and environmental processes that occur where coastal rivers empty into estuaries. Although recognized as an important feature where significant thicknesses of coastal deposits accumulate, application of current facies models that rely upon specific sedimentary structures, marine fossils, and ichnofacies are still unable to accurately predict the depositional position of ancient FTTZ strata. Building upon previous studies that correlated sediment geochemistry with paleosalinity, we plan to use a multi-pronged approach on both modern and ancient deposits consisting of lithofacies and palynological analysis, C/S, and major and trace element geochemistry. Major element data was collected via XRF analysis of fused discs while trace elements were collected via LA-ICP-MS on glass remnants left over from the fluxing process. Principal component analysis was used to determine the most useful set of variables from the modern samples for maximum category separation among each FTTZ subenvironment, and then those variables are used to create discriminate diagrams for use with ancient strata.

Modern samples consist of 26 sediment cores and surface grab samples with accompanying salinity measurements collected in the Ogeechee River estuary in Georgia, USA. Consisting primarily of channel margin mud and clay, longitudinal and point bar sand, and mud and sand collected from within the channel, our sampling traverse covers 50 km of the FTTZ from the mouth of the estuary through the fluvial reach. Ancient samples are from the Lower-Middle Pennsylvanian lower Cherokee Group in the Forest City Basin in south-central Iowa. These strata sit in pre-Pennsylvanian incised valleys that characterize the basal unconformity of the Absaroka megasequence, record an overall rise in eustatic sea level, and constitute a relatively undocumented FTTZ in the rock record. Eighteen cores of the Lower Cherokee Group have been described and sampled. This model, developed in the modern and tested in the ancient, will afford greater predictive power for paleogeographic reconstructions, hydrocarbon exploration, and tidal modeling.